1. Field of the Invention
This invention relates to a shift control device for an automatic transmission for use in an automotive vehicle or other vehicle.
2. Description of the Related Art
In a shift control device for an automatic transmission, each shift change is accomplished by engagement and/or disengagement of a combination of multiple friction engaging elements (clutches, brakes).
Whereas, in the relationship between two friction engaging elements, one operated on the engaging side and the other operated on the releasing or disengagement side, the shift change between these elements must change over timely between the supply of hydraulic pressure and the exhaust of hydraulic pressure so as to reduce the shift shock. Therefore, an oil pressure control circuit for the shift control device is provided with an accumulator and a shift timing valve which opens a drain port on the releasing side in response to an increase of the hydraulic pressure of the engaging side.
A Control is also proposed for the sake of further reducing shift shock. That is to say, a timing solenoid valve is connected to the shift timing valve and controlled by an engine operating condition. The timing solenoid valve controls the exhaust of the hydraulic pressure from the drain port of the shift timing valve.
FIG. 5 shows an oil pressure control circuit of the foregoing underlap control type.
A 2-3 shift valve 10 is provided with a plural ports 12, 14, 16, 18, 20 and a spring member 22. The ports 12 and 14 as a first port supply a line pressure PL into a C2 clutch as a first friction engaging element via oil lines 30 and 32. The ports 16 and 18 as a second port supply a hydraulic pressure to a B1 brake as a second friction engaging element into a shift timing valve 40 via oil lines 34 and 36. The port 20 is located at one end of the 2-3 shift valve 10 and is connected with a solenoid valve 60 via an oil line 38. The spring member 24 is disposed in a chamber 24 which is located at the other end of the 2-3 shift valve 10 and urges a spool 26 in the upward direction as shown in the drawing.
The shift timing valve 40 is provided with timing port 42, 44, 46, 48, 50 and a first spring member 52. The first timing port 42 is in communication with the port 14 of the 2-3 shift valve 10 via the line 32. The second timing port 44 is in communication with the port 18 of the 2-3 shift valve 10 via the line 36. The third port 46 is connected to the drain side (not shown) via an orifice. The fourth port 48 is also connected to the drain side (not shown). The fifth port 50 is located at one end of the shift timing valve 40 and is connected with a timing solenoid valve 62 via a line 58. The first spring member 52 is disposed in a chamber 54 which is located at the other end of the shift timing valve 40 and urges a spool 56 in the downward direction as shown in the drawing. The timing solenoid valve is controlled by an electric control device (not shown). Accumulators 64, 66 are provided on lines 32, 34.
When the solenoid valve 60 which is of the normal open type is operated by a shift signal (not shown) which is outputted from the electric control device (not shown), a shift signal pressure Pv is supplied to the port 20 of the 2-3 shift valve 10 and acts to bias the spool 26 in the downward direction, as shown in the drawing, against a spring force of the spring member 22. Thereby, the line pressure PL is supplied to the C2 clutch with the result that the ports 12 and 14 communicate with each other. Also, the spool 56 of the shift timing valve 40 is moved in the upward direction, as shown in the drawing, with the result that a direct clutch pressure PC2 is supplied to the first timing port 42 via the line 32. Therefore, the engaging hydraulic pressure of the B1 brake is released with the result that the ports 16 and 18 communicate with each other. Correspondingly, a second brake pressure PB1 is supplied to the second timing port 44 via the line 36, and further the engaging hydraulic pressure of the B1 brake is drained from the third timing port 46 with the result that the second timing port 44 and the third timing port 46 communicate with each other. Accordingly, the 2-3 shift change is accomplished smoothly, without shift shock, by the shift timing valve 40.
The direct clutch pressure PC2 is increased corresponding to a characteristic of the accumulator 64, as shown by line 100 in FIG. 3-c, and is supplied to the direct clutch pressure PC2 and the first timing port 42 of the shift timing valve 40. As far as the C2 hydraulic pressure (the servo pressure) which is supplied to the first timing port 42 is less than a predetermined pressure P1, the fourth timing port 48 is maintained in a closed position. Thereby, the B1 hydraulic pressure which is supplied to the second timing port 44 is drained from only the third timing port 46 as shown by line 102 in FIG. 3-c.
When the timing solenoid valve, which is normally open, is operated by the engine condition (i.e. engine output), a solenoid signal pressure PS is supplied to the fifth timing port 50 of the 2-3 shift timing valve 40 as shown by point A in FIG. 3-c. The fourth timing port 48 is therefore opened. Correspondingly, the foregoing B1 hydraulic pressure is released from the fourth timing port 48 and is decreased as shown by line 104 in FIG. 3-c. As a result, the C2 clutch is not engaged when the B1 second brake is released. Namely there is an underlap condition as shown by line 106 in FIG. 3-c. Thereafter the C2 hydraulic pressure is increased. The 2-3 shift change is accomplished due to the C2 clutch being engaged.
However, in the above-mentioned Related Art, when the timing solenoid valve 62 has a valve-stick condition, as shown in FIG. 4-b, the solenoid signal pressure PS of the timing solenoid valve 62 is continuously supplied to the first timing port 42 of the shift timing valve 40. Thereby, the B1 hydraulic pressure is released from the third and fourth ports 46, 48 together when the 2-3 shift change is performed. Accordingly, the shift shock is increased because the engine output is abruptly increased (i.e. the engine output is large.). Also, the friction engaging elements are damaged due to premature wear by the result.